Extrusion die membrane assembly

ABSTRACT

A membrane assembly for an extrusion die includes a membrane having a deformable main portion and first and second mounting arms integral with the main portion and disposed on opposite sides thereof. The main portion includes a first face and the mounting arms extend away from the first face. The main portion, the mounting arms and a second face opposite the first face form a recess on a side of the main portion opposite the first face and a backing member is disposed in the recess between the mounting arms.

FIELD OF THE INVENTION

The present invention relates generally to extrusion dies, and more particularly to apparatus for controlling material flow through the die.

BACKGROUND ART

Extrusion processes have long been used to produce sheets or webs of formable material, such as thermoplastic. These processes generally involve forcing a viscous material through a die typically comprising an inlet, a cavity, and an exit. As the material passes through the die, it is often necessary to influence the flow to obtain a product at the die exit having desirable characteristics.

Past methods of influencing material flow have included the use of a flow control device mounted in a channel within a die cavity, such as a restrictor bar, a flexible membrane, or another insert. Since most extrusion processes operate under high pressure, leakage of material into spaces between the insert and the channel is a significant concern. Build-up of material in the channel can prevent proper flow control and result in burning of material therein. Hence, the channel and insert must be designed and machined to tolerances that will ensure a tight sealing fit and thereby prevent leakage.

The assignee of the instant application has offered for sale a die having a flexible membrane mounted by spaced mounting members in correspondingly-shaped tapered channels in the die cavity. Bolts extend into the mounting members and are tightened to cause the members to seal against walls forming the tapered channel. End surfaces of the membrane are clamped between body members of the die to form a seal between the die and the membrane.

While this arrangement of elements is effective to accomplish flow control, some drawbacks are encountered. Specifically, the installation process is complicated and expensive. Further, machining of the tapered channels is difficult to accomplish with the required degree of precision and is not well suited (due to the channel configuration) to large-scale production techniques. In addition, re-working of such a die would likely result in a substantial amount of refitting of the flow control device in the channels. Manufacture of an aftermarket flow control device without the original tooling would also be problematic. Furthermore, clamping the end surfaces of the membrane can restrict adjustment of the membrane which, in turn, can make it more difficult to achieve a desired flow control.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a membrane assembly for an extrusion die includes a membrane having a deformable main portion and first and second mounting arms integral with the main portion and disposed on opposite sides thereof wherein the main portion includes a first face. The mounting arms extend away from the first face and the main portion, the mounting arms and a second face opposite the first face form a recess on a side of the main portion opposite the first face. A backing member is disposed in the recess between the mounting arms.

Preferably, the backing member includes a channel therein adjacent the main portion and the membrane assembly further includes an adjustment member disposed in the channel in abutment with the main portion. Also preferably, the adjustment member is serrated and includes a plurality of alternating lands and grooves wherein each land includes a substantially planar bearing surface. Still further, a movable member is preferably in contact with one of the bearing surfaces and the movable member comprises an adjustment screw which is rotatable to move the bearing surface and thereby deform the main portion. Also, the first face may be concave in shape.

Still further in accordance with the preferred embodiment, the second face includes a substantially planar central portion, first and second curved portions disposed on either side of the central portion, a first shoulder adjacent the first curved portion and a second shoulder adjacent the second curved portion. In addition, the main portion may include offset portions of differing thicknesses. Also, the backing member may include a first plurality of alternating lands and grooves and the main portion may include a second plurality of alternating thick and thin portions wherein the lands are aligned with the thick portions and the grooves are aligned with the thin portions.

In accordance with a further aspect of the present invention, an extrusion die includes a pair of body portions forming a passage therebetween and a membrane assembly is disposed in one of the body portions. The membrane assembly includes a membrane having a deformable main portion disposed in the passage and first and second mounting arms integral with the main portion and disposed on opposite sides thereof The main portion includes a first face and the mounting arms extend away from the first and the main portion. The mounting arms and a second face form a recess on a side of the main portion opposite the first face. The membrane assembly further includes a backing member disposed in the recess between the mounting arms.

Other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a cross-sectional view of a prior art flow control device disposed within a die;

FIG. 2 comprises a cross-sectional view of a first embodiment of a flow control device disposed within a die cavity;

FIG. 3 comprises an enlarged end elevational view of the flow control device of FIG. 2;

FIG. 4 comprises a perspective view of the flow control device of FIG. 2;

FIGS. 5 and 6 are fragmentary, enlarged, cross-sectional views illustrating the installation of the flow control device of FIG. 2;

FIG. 7 comprises an exploded perspective view of a sealing apparatus for the flow control device of FIGS. 2-6;

FIG. 8 comprises a cross-sectional view of a further embodiment of a flow control device;

FIG. 9 comprises a cross-sectional view of a yet another embodiment of a flow control device disposed within a die cavity;

FIG. 10 comprises a plan view of an extrusion die membrane of the flow control device of FIG. 9,

FIG. 11 comprises a side elevational view of the extrusion die membrane of FIG. 10;

FIG. 12 comprises a cross-sectional view of the extrusion die membrane taken generally along the lines 12—12 of FIG. 10;

FIG. 13 is a view similar to FIG. 2 illustrating a membrane assembly comprising an embodiment of the present invention as installed in a die;

FIG. 14 is an end elevational view of the membrane assembly of FIG. 13,

FIG. 15 is a fragmentary trimetric view of the membrane assembly of FIG. 13,

FIGS. 16 and 17 are isometric views of the membrane of FIG. 13,

FIG. 18 is a fragmentary isometric view of the backing member of FIG. 13;

FIGS. 19 and 20 are fragmentary plan and side elevational views, respectively, of the adjustment member of FIG. 13; and

FIG. 21 is a sectional view of the adjustment member taken generally along the lines 21—21 of FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a prior art flow control device 10 is disposed within an extrusion die 12. The flow control device 10 is disposed within a die cavity 14 having a first tapered channel 16 and a second tapered channel 18. In the device illustrated in FIG. 1, sidewalls 17 a, 19 a in part defining the channels 16, 18 are disposed at angles of other than 90° with respect to base surfaces 17 b, 19 b, respectively. The flow control device 10 comprises a flexible membrane 20, a first mounting member 22, and a second mounting member 24. The flexible membrane 20 is disposed between and joined to the mounting members 22, 24. Furthermore, the flexible membrane 20 is movable in a direction perpendicular to the direction of flow and has end surfaces (not shown) that extend beyond the mounting members 22, 24.

Each of the mounting members 22, 24 includes a base 26 and two sides 28, 30 adjacent the base 26. The two sides 28, 30 of the mounting member 22 are complementarily-shaped with respect to the sidewall 17 a and a sidewall 17 c forming the channel 16. Similarly, the two sides 28, 30 of the mounting member 24 are complementarily-shaped with respect to the sidewall 19 a and a sidewall 19 c forming the channel 18. Bolts 32 extending through bores in the die and into the mounting members 22, 24 secure the flow control device 10 within the tapered channels 16, 18. One or more additional bolts 33 extend through threaded bores 34 in the die and push against a stiffener plate 35 which is disposed in contact with a back surface of the membrane 20.

The stiffener plate 35 is provided to permit adjustment of the position of the flexible membrane 20 according to the desired flow characteristics and end product. The end surfaces of the flexible membrane 20 are clamped between upper and lower body members 12 a, 12 b, of the die 12 to form a seal between the die 12 and the flexible membrane 20.

As discussed above, disadvantages are encountered with the design of the flow control device 10 shown in FIG. 1 including the production, re-working, and aftermarket design problems noted above.

Shown in FIGS. 2 through 7 is one embodiment of a mounting apparatus which overcomes the problems encountered with the prior art flow control device 10 of FIG. 1. With specific reference to FIGS. 2 through 4, a flow control device 40 is disposed within a die cavity 42 of a flow handling apparatus, shown here as an extrusion die 44. The flow control device 40 is disposed within a channel 46 defined by a base surface 48, a first side surface 50, and a second side surface 52. Each of the side surfaces 50, 52 is normal to the base surface 48.

The flow control device 40 has a main body 54 and an auxiliary body 56 joined to and extending away from the main body 54. The main body includes a first side portion 58 and a second side portion 60. When disposed in the channel 46, the auxiliary body 56 extends into contact with the base surface 48 of the channel 46. Furthermore, the first side portion 58 is adjacent to the first side surface 50 and the second side portion 60 is adjacent to the second side surface 52.

The flow control device 40 is held in place in the channel 46 by force-transmitting members 62 which may be bolts, screws or other fasteners. The force-transmitting 62 are disposed in bores 64 in the extrusion die 44 and further extend into threaded bores 65 in the side portions 58, 60 of the flow control device 40. Typically, the spacing between the force-transmitting members 62 is between about 0.75 inches and about 2.5 inches.

The flow control device 40 further includes a flexible membrane 68 joined to and disposed between each of the side portions 58, 60 on a face opposite the auxiliary body 56. In a first embodiment, the membrane 68 is welded or otherwise joined to the main body 54. A number of force transmitting members 70, which may be threaded bolts, screws, or other fasteners, are disposed in bores 72 in the die 44 and extend through threaded portions 73 and push against at least one, and preferably two (or more) stiffener plates 74. The stiffener plates 74 in turn serve as a rigid member or support for the flexible membrane 68. The force transmitting members 70 may be threaded into or out of the threaded portions 73 to change the profile of the membrane 68 and, thus, change the flow of material in the die cavity 42. For example, when one or more of the force transmitting members 70 is (or are) threaded into the threaded portions 73, the stiffener plates 74 move the membrane 68 into the flow of material through the die cavity 42. Typically, the spacing between the force-transmitting members 70 is between about 0.75 inches and about 2.5 inches.

Each of the side portions 58, 60 further includes a sealing surface 66, 67, respectively, and a channel 75, 76, respectively, in which a sealing element 77, 78 may be disposed. The sealing element 77, 78 preferably is constructed of a formable material capable of operation in a hot and corrosive environment, such as a metal- or carbon-impregnated Teflon™ material or a metal material.

As shown in FIG. 5, before the members 62 are threaded into the bores 65 to exert forces on the side portions 58, 60, a gap 79 exists between the side portions 58, 60 and the side surface 50, 52. (This gap is shown in exaggerated fashion in FIG. 5 for the sake of explanation.)

However, as seen in FIG. 6, when the members 62 are threaded into the bores 65, bending moments are introduced in the side portions 58, 60, causing such portions to deflect about the auxiliary body 56 (which is in contact with the base surface 48, as noted previously). Arrows 80 shown in FIG. 6 illustrate the movement of the side portion 60 during tightening of the members 62. This movement eventually causes each sealing surface 66, 67 to contact each side surface 50, 52, respectively, over areas of the surfaces 50, 52, 66, 67. Further, each sealing element 77, 78 is compressed within each channel 75, 76 and bears against the side surface 50, 52, respectively. Thus, a tight seal forms between the side portions 58, 60 and the side surfaces 50, 52 to prevent leakage into the space between the main body 54 and the channel 46.

Shown in FIG. 7 is the flow control device 40 disposed within the die 44 and a sealing apparatus 90 disposed adjacent end portions 92 of the flow control device 40 and the die 44. The sealing apparatus 90 forms a tight seal between a built-up portion 93 of the flexible membrane 68 and end plates 94 of the die 44. Each sealing apparatus 90 includes one or more backing plates 96 and a gasket 98 disposed in side-by-side relationship in a recess 99 in each end plate 94. The gasket 98 is made of any suitable material capable of withstanding the die environment. One or more force transmitting members 100, which may be threaded bolts or screws, are disposed in bores 102 in the end plate 94 and push against the backing plate 96. The backing plate 96, in turn, pushes against the gasket 98 and causes the gasket 98 to seal against the built-up portions 93 of the flexible membrane 68. The force causing the gasket 98 to move is preferably directed transversely to the direction of movement of the flexible membrane 98.

Preferably, as seen in FIG. 3, each built-up portion may comprise a curved member 103 which is preferably (although not necessarily) rectangular in cross-section and which is welded or otherwise joined to or formed with the membrane 68 contiguous with an edge 104 of the membrane 68. Also preferably, each curved member 103 is sufficiently thin and narrow to permit adequate adjustability of the membrane 68, and further has a coefficient of thermal expansion (CTE) similar or identical to the CTE of the membrane 68. The curved members 103 present flat sealing surfaces to the gaskets 98 so that the gaskets 98 are not damaged by the thin edges of the membrane 68 and so that the ends of the membrane 68 can slide over the gaskets 68 during profile adjustment.

The sealing apparatus 90 prevents leakage of material between the die 44 and the flexible membrane 68. Furthermore, this sealing arrangement does not significantly restrict the movement and adjustment of the flexible membrane 68 and, thus, does not significantly interfere with the ability to influence material flow past the flow control device 40.

As seen in FIG. 8, the membrane 68 can be retained within opposed slots 110, 112 in the flow control device 40. The slots may be formed by cap members 114, 116 which are held in position overlying channeled portions 118, 120 of the main body 54 by bolts or other fasteners (not shown). Alternatively, the cap members 114, 116 may be welded to the main body 54.

Dimensions for the flow control device 40 and the sealing apparatus will vary depending on the flow characteristics of the material flowing through the die cavity 42 and the various process parameters.

In summary, the flow control device 40 is an improvement over the above-described prior art device 10. Specifically, the flow control device 40, unlike the prior art device 10, is not tapered, and hence is more easily manufactured to the required degree of precision. Additionally, manufacturing tolerances can be relaxed as compared with the prior art device 10 because a range of gap dimensions can be eliminated by the movement of the side portions 58, 60 into engagement with the side surfaces 50, 52. Furthermore, installation of the device 40 is simplified and cheaper since it is easier to machine the channel 46 owing to the fact that it has non-tapered sidewalls. Still further, manufacture of an aftermarket flow control device without the original tooling is no longer a problem. Additionally, the sealing apparatus does not significantly restrict movement of the flexible membrane and, thus, does not interfere with flow control.

Referring now to FIGS. 9-12, which show another embodiment, a flow control device 210 is disposed within a die cavity 212 of a flow handling apparatus, shown here as an extrusion die 214. Although not described in detail below, and not shown in detail in FIGS. 9-12, the embodiment shown in FIGS. 9-12 includes a side sealing apparatus similar to that shown in FIGS. 4 and 7. The flow control device 210 is disposed within a channel defined by a base surface 218, a first side surface 220, and a second side surface 222 Each of the side surfaces 220, 222 is normal to the base surface 218.

The flow control device 210 includes a flexible membrane 238 and a membrane holder 211. The holder 211 has a main body 224 and an auxiliary body 226 joined to and extending away from the main body 224 The main body 224 includes a first side portion 228 and a second side portion 230. When disposed in the channel 216, the auxiliary body 226 extends into contact with the base surface 218 of the channel 216. Furthermore, the first side portion 228 is adjacent to the first side surface 220 and the second side portion 230 is adjacent to the second side surface 222.

The flow control device 210 is held in place in the channel 216 by force-transmitting members 232 which may be bolts, screws, or other fasteners. The force-transmitting members 232 are disposed in bores 234 in the extrusion die 214 and further extend into threaded bores 236 in the side portions 228, 230 of the main body 224.

Typically, the spacing between the force-transmitting members 232 is between about 0.75 inches and about 2.5 inches. As in the embodiment of FIGS. 2-8, the force-transmitting members 232 are tightened to bend the side portions 228, 230 of the main body 224 into sealing contact with the side surfaces 220, 222. Sealing elements (not shown) similar or identical to the sealing elements 77, 78 may be provided in channels (also not shown) similar or identical to the channels 75, 76 to assist in sealing, if desired.

The flexible membrane 238 is joined to and disposed between each of the side portions 228, 230 on a face opposite the auxiliary body 226. In the embodiment of FIGS. 9-12, the membrane 238 is welded or otherwise joined to the main body 224. As shown in FIGS. 10-12, the flexible membrane 238 includes a horizontal portion 248, a first curved portion 243 adjacent the horizontal portion 248, and a second curved portion 247 adjacent the horizontal portion 248 opposite the first curved portion 243. The curved portions 243, 247 typically have radii of curvature of about 1 inch to about 3 inches. In a preferred embodiment, each radius of curvature is about 2½ inches. The membrane 238 also includes a first offset (or inclined) portion 242 adjacent the first curved portion 243 and a second offset (or inclined) portion 246 adjacent the second curved portion 247.

The first and second offset portions 242, 246 terminate at first and second edges 240, 244. Typically, the offset portions 242 and 246 are offset from the horizontal portion 248 by an angle of between about 5 degrees and about 15 degrees. In a preferred embodiment the offset angle is about 10 degrees. As seen in FIGS. 11 and 12, the horizontal portion 248 includes a plurality of evenly-spaced, substantially flat-bottomed grooves 250 disposed between land portions 251. The flat-bottomed grooves 250 are positioned to allow more flexibility across the die cavity 212. Typically, the grooves 250 are spaced apart by about ½ inch to about 1½ inches. Preferably, the spacing is about ¾ inches. The horizontal portion 248 has an effective adjustment length parallel to the flow of extrudate through the die preferably between about 1 inch and about 1½ inches. In a highly preferred embodiment, this length is about 1¼ inches. This length allows greater adjustability of the flow of extrudate through the die, compared to other dies having a shorter effective adjustment length.

As shown in FIG. 9, a number of force transmitting members 252, which may be threaded bolts, screws, or other fasteners, are disposed in bores 254 in the die 214 and extend through threaded portions 256 and push against the top surface 258 of the membrane 238. The force transmitting members 252 may be threaded into or out of the threaded portions 256 to bear against the land portions 251 and change the profile of the membrane 238 and, thus, change the flow of material in the die cavity 212. For example, when one or more of the force transmitting members 252 is (or are) threaded into the threaded portions 256, the membrane 238 is deflected into the flow of material through the die cavity 212. Typically, the spacing between the force-transmitting members 252 (and the spacing between centers of the land portions 251) is between about 0.75 inches and about 2.5 inches. In a preferred embodiment, as shown in FIG. 9, the edges 240, 244 of the membrane are located substantially on the same line between the die entrance and die exit. This membrane design allows the extruded material to enter and exit the extrusion die land channel in substantially coplanar fashion, i.e., at the same level when the die is oriented as shown in FIG. 9.

The membrane 238 preferably comprises a single piece of material. For example, the membrane may be machined from a solid piece of stainless steel, such as 400 series stainless steel. Further, in a preferred embodiment both the membrane 238 and holder 211 are formed from the same material. After the membrane 238 and holder 211 are fabricated, the membrane and holder are preferably joined together by welding and then hardened simultaneously by a heat-treatment process. By fabricating and preparing the membrane 238 and holder 211 in this manner, the membrane 238 and holder 211 will have the same coefficient of thermal expansion. Consequently, the weld holding these parts together will not be weakened by differential thermal expansion as the membrane and holder increase in temperature during use. Separate stiffener plates 35 are not required for this embodiment because the membrane 238 has a stiffened area 262 integrally included in the membrane 238, as shown in FIGS. 10-12. Accordingly, the stiffened area 262 is machined in the desired location of the membrane 238, in contrast to other designs that limit the stiffened area to the area between adjustment screws, along with the area under the screws.

In a preferred embodiment, the holder 211 and membrane 238 are fabricated from 400 series stainless steel, such as Carpenter Custom 455® stainless steel, which is a martensitic age-hardenable stainless steel. Specifically, Carpenter Custom 455® stainless steel in condition H 900 has been found suitable for the present invention. For this particular steel, high strength levels are achieved by employing a precipitation-hardening treatment consisting of the steps of heating the steel to a selected temperature between 900° F. and 1050° F., holding the steel at the selected temperature for four hours, and air cooling the steel.

Dimensions for the membrane 238 and holder 211 will vary depending on the flow characteristics of the material flowing through the die cavity 212 and the various process parameters.

Referring now to FIGS. 13-21, which show an embodiment of the present invention, a flow control device in the form of a membrane assembly 310 is disposed within a die body 312 of a flow handling apparatus, shown here as an extrusion die 314. Although not described in detail below, and not shown in detail in FIGS. 13-21, the embodiment of FIGS. 13-21 includes a side sealing apparatus similar to that shown in Figs. 4 and 7. The membrane assembly 310 is disposed within a channel 316 defined by a base surface 318, a first side surface 320 and a second side surface 322. Each of the side surfaces 320, 322 is normal to the base surface 318.

The membrane assembly 310 includes a flexible membrane 338 having a deformable main portion 340 and first and second mounting arms 342, 344 integral with the main portion 340 and disposed on opposite sides thereof. The main portion 340 includes a first face 346 which is generally concave in shape and the mounting arms 342, 344 extend away from the first face 346. The main portion 340, the mounting arms 342, 344 and a second face 348 opposite the first face 346 form a recess on a side of the main portion 340 opposite the first face 346. A backing member 350 is disposed in the recess and, as seen in FIG. 18, includes a plurality of apertures 352 extending though a central section 354 thereof (only three of the apertures are indicated with the reference numeral 352 in FIG. 18). The backing member 350 is secured to the mounting arms 342, 344 by bolts 359 (FIG. 14) or other fasteners.

An adjustment member 360 is disposed in a channel 362 of the backing member 350. As seen in FIGS. 19-21, the adjustment member 360 is serrated and includes a plurality of alternating lands 364 and grooves 366 (again, only three of which are identified in FIGS. 19-21). Each land 364 has a generally planar bearing surface 368 and a plurality of movable members 370 in the form of threaded adjustment screws or bolts are disposed in bores 372 in the die body 312 and extend though the apertures 352 into contact with the bearing surfaces 368. Each movable member 370 is axially extended or retracted by turning an associated head 374 of the member 370, thereby causing the movable member 370 to thread through a nut 377 which is restrained against axial and rotational movement by a U-shaped bracket 376 bolted to the die body 312.

The membrane assembly 310 is held in place in the channel 316 by force-transmitting members 382 which may be bolts, screws, or other fasteners. The force-transmitting members 382 are disposed in bores 384 in the die body 312 and further extend into threaded bores 386 (FIG. 14) in the mounting arms 342, 344. As in the embodiment of FIGS. 2-8, the force-transmitting members 382 are tightened to bend the mounting arms 342, 344 into sealing contact with the side surfaces 320, 322. Sealing elements 388 similar or identical to the sealing elements 77, 78 may be provided in channels similar or identical to the channels 75, 76 to assist in sealing, if necessary or desirable. In addition, pullback screws or bolts 389 are threaded into bores 387 (FIG. 14) in the mounting arm to provide further sealing on the downstream side of the membrane. The pullback screws 389 apply force to the first mounting arm 342 further bringing it into sealing contact with side surface 320.

As shown in FIGS. 13 and 14, the flexible membrane 338 includes a substantially planar central portion 390, a first curved portion 392 adjacent the central portion 390 and a second curved portion 394 adjacent the central portion 390 opposite the first curved portion 392. The curved portions 392, 394 typically have radii or curvature of about 1 to 3 inches. In the preferred embodiment, each curved portion 392, 394 has a radius of curvature of about 2½ inches. The membrane 338 also includes a first shoulder 396 adjacent the first curved portion 392 and a second shoulder 398 adjacent the second curved portion 394. As seen in FIG. 16, the second face 348 of the main portion 340 includes a plurality of evenly-spaced, substantially flat-bottomed grooves 400 disposed between offset land portions 402, similar to the embodiment of FIG. 10. The land portions 402 are aligned with the lands 364 of the adjustment member 360 whereas the grooves 400 are aligned with the grooves 366 of the adjustment member 360. Further, in the preferred embodiment the central portion 390 has an effective adjustment length parallel to the flow of extrudate through the die of between about 1 inch and about 1½ inches. In a highly preferred embodiment, this length is about 1-¼ inches.

Referring again to FIGS. 13-15, when the movable members 370 are rotated and thereby extended or retracted the profile of the membrane 338 is changed, in turn changing the flow of material in the die cavity. For example, when one or more of the movable members 370 is (or are) extended, the membrane 338 is defected into the flow of material through the die. Typically, the spacing between the movable members 370 (and the spacing between centers of the lands 364 and the centers of the land portions 402) is between about 0.75 inches and about 2.5 inches. In a preferred embodiment of the invention, as shown in FIG. 13, the first and second shoulders 396 and 398 of the membrane 338 include outer surfaces located substantially on the same line between the die entrance and die exit. This membrane design allows the extruded material to enter and exit the extrusion die land channel in substantially coplanar fashion, i.e., at the same level when the die is oriented as shown in FIG. 13.

As noted above, the membrane 338 preferably comprises a single piece of material. For example, the membrane may be machined from a solid piece of stainless steel, such as 400 series stainless steel. In this manner the weld required in the previous embodiments is eliminated. Further, the membrane 338, the backing member 350 and the adjustment member 360 may all be formed from the same material and hardened simultaneously by a heat-treatment process. By fabricating the membrane 338, the backing member 350 and the adjustment member 360 in this manner, these members will have the same coefficient of thermal expansion.

In the preferred embodiment, the membrane 338, the backing member 350 and the adjustment member 360 are fabricated from 400 series stainless steel, such as Carpenter Custom 455® stainless steel, which is a martensitic age-hardenable stainless steel. Specifically, Carpenter Custom 455® stainless steel in condition H 900 has been found suitable for the present invention. For this particular steel, high strength levels are achieved by employing a precipitation-hardening treatment consisting of the steps of heating the steel to a selected temperature between 900° F. and 1050° F., holding the steel at the selected temperature for four hours, and air cooling the steel.

Dimensions for the membrane 338, the backing member 350 and the adjustment member 360 will vary depending on the flow characteristics of the material flowing through the die cavity and the various process parameters.

The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications within the scope of the invention will be apparent to those skilled in the art. 

What is claimed is:
 1. A membrane assembly for an extrusion die, comprising: a flexible membrane having a deformable main portion and first and second mounting arms integral with the main portion and disposed on opposite sides thereof wherein the main portion includes a first face and a second face opposite the first face and the mounting arms extend away from the first face and wherein the mounting arms and the second face form a recess on a side of the main portion opposite the first face; fasteners extending through a portion of the extrusion die and into bores in the mounting arms to mount the flexible membrane in a channel of the extrusion die; and a backing member disposed in the recess between the mounting arms and secured to both mounting arms in direct contact therewith.
 2. The membrane assembly of claim 1, wherein the backing member includes a channel therein adjacent the main portion and further including an adjustment member disposed in the channel in abutment with the main portion.
 3. The membrane assembly of claim 2, wherein the adjustment member is serrated and includes a plurality of alternating lands and grooves.
 4. The membrane assembly of claim 3, wherein each land includes a substantially planar bearing surface.
 5. The membrane assembly of claim 4, further including a movable member in contact with one of the bearing surfaces.
 6. The membrane assembly of claim 5, wherein the movable member comprises an adjustment screw which is rotatable to move the bearing surface and thereby deform the main portion.
 7. The membrane assembly of claim 1, wherein the first face is concave in shape.
 8. The membrane assembly of claim 1, wherein the flexible membrane further includes a substantially planar central portion, first and second curved portions disposed on either side of the central portion, a first shoulder adjacent the first curved portion and a second shoulder adjacent the second curved portion.
 9. The membrane assembly of claim 1, wherein the main portion includes a groove of a first thickness and a land of a second thickness different than the first thickness.
 10. The membrane assembly of claim 1, further including an adjustment member disposed in a channel of the backing member wherein the adjustment member includes a first plurality of alternating lands and grooves and the main portion includes a second plurality of alternating lands and grooves and wherein the lands of the adjustment member are aligned with the lands of the main portion and the grooves of the adjustment member are aligned with the grooves of the main portion.
 11. An extrusion die, comprising: a pair of body portions forming a passage therebetween; a membrane assembly disposed in one of the body portions and including a flexible membrane having a deformable main portion disposed in the passage and first and second mounting arms integral with the main portion and disposed on opposite sides thereof wherein the main portion includes a first face and a second face opposite the first face and the mounting arms extend away from the first face and wherein the mounting arms and the second face form a recess on a side of the main portion opposite the first face and wherein the membrane assembly further includes a backing member disposed in the recess between the mounting arms and secured to both mounting arms in direct contact therewith; and fasteners extending through the one body portion and into bores in the mounting arms to mount the flexible membrane in a channel of the one body portion.
 12. The extrusion die of claim 11, wherein the backing member includes a channel therein adjacent the main portion and wherein the extrusion die further includes an adjustment member disposed in the channel in abutment with the main portion.
 13. The extrusion die of claim 12, wherein the adjustment member is serrated and includes a plurality of alternating lands and grooves.
 14. The extrusion die of claim 13, wherein each land includes a substantially planar bearing surface.
 15. The extrusion die of claim 14, further including a plurality of movable members in contact with the bearing surfaces.
 16. The extrusion die of claim 15, wherein each movable member comprises an adjustment screw which is rotatable to move one of the bearing surface thereby deform the main portion.
 17. The extrusion die of claim 16, wherein the first face is concave in shape.
 18. The extrusion die of claim 11, wherein the flexible membrane further includes a substantially planar central portion, first and second curved portions disposed on either side of the central portion, a first shoulder adjacent the first curved portion and a second shoulder adjacent the second curved portion.
 19. The extrusion die of claim 11, wherein the main portion includes a groove of a first thickness and a land of a second thickness different than the first thickness.
 20. The extrusion die of claim 11, further including an adjustment member disposed in a channel of the backing member wherein the adjustment member includes a first plurality of alternating lands and grooves and the main portion includes a second plurality of alternating lands and grooves and wherein the lands of the adjustment member are aligned with the lands of the main portion and the grooves of the adjustment member are aligned with the grooves of the main portion. 